In electrostatographic printing machines an electrostatic charge image is formed on a dielectric recording member that may be a photoconductive dielectric recording member which after being uniformly charged is image-wise exposed to conductivity-increasing radiation producing thereby a "direct" or "reversal" toner-developable charge pattern on the recording member.
In one type of development of electrostatic charge images a two-component developer is used in which coloring particles, called toner particles, are mixed with larger magnetizable carrier particles, to which the toner particles adhere by electrostatic attraction force. The electrostatic charge of the toner and carrier particles is obtained triboelectrically by agitation. The charge sign of the toner particles is opposite to the charge sign of the carrier particles. In most commercial copying and printing machines negatively charged toner is used.
It is common practice to apply the toner-carrier mixture to the surface carrying the electrostatic charge image by means of a developing unit wherein toner and magnetizable carrier particles are mixed and a layer of the toner-carrier mixture, referred to herein as "developer" is picked up by an applicator such as a rotating sleeve or drum having magnets inside, forming a so-called magnetic brush on a "magnetic roller".
On rotating the magnetic roller, the toner particles still adhering to the magnetically attracted carrier particles are brought into a developing zone wherein the toner particles are separated from the carrier particles by the electrostatic attraction forces of the electrostatic latent image to be developed and transfer to the latent electrostatic charge image. A developing bias voltage of suitable polarity applied between the magnetic brush and the recording member to be developed decides whether the development is a "direct" or "reversed" development.
The magnetic brush, from which toner particles are removed during each revolution, to be taken up by the developed electrostatic charge image, has to be supplied with fresh toner-carrier mixture. This is normally done by an agitator projecting or scooping up toner-carrier mixture onto the magnetic roller from a housing for holding the developer. The partly exhausted developer is returned to the bulk of developer contained in the housing and has to be thoroughly mixed timely with freshly added toner to keep the toner-carrier weight ratio within acceptable limits for obtaining consistent development results.
It is known, for example from U.S. Pat. No. 4,576,466 (Fukuchi/Konishiroku Photo Industry), to provide a toner supply hopper on one side of the width of an area to be developed. The hopper supplies toner in response to a toner replenishment signal when the density of the image pattern becomes too low. The toner thus supplied is conveyed along a gutter-like conveyance guide or receptacle in the direction of the width of the area to be developed by means of a spiral screw. The toner is replenished through toner dropping holes provided substantially under the conveyance guide.
In order to obtain even distribution of developer particles, a pair of screw members has been proposed as described in U.S. Pat. No. 4,576,466. The invention disclosed therein deals with a solution to the problem of obtaining a sufficiently thoroughly mixed toner-carrier developer composition in the developing unit. The developing system proposed includes agitating means arranged between the secondary screw member means and the applicator, the agitating means including means for scooping up developer to fly onto the primary screw member to mix with developers supplied from a supply means.
In PCT patent application W0 93/09475 (Fujitsu Limited) a developing unit is described in which two screw members are provided to distribute a two-component developer across the unit. The two screw members are separated by a partition which extends across a central portion of the unit, but allows communication between the two screw members at each end thereof, enabling circulation of at least part of the developer in the unit.
Although the above described developing systems already gives a substantial improvement in toner-carrier mixing and developing reproducibility, further improvements can be made with regard to mixing efficiency and the prevention of damming up toner-carrier mixture at the downstream end of the secondary screw member.